Tree kangaroos have a very extensive diet. This diet includes bark, sap, and flowers. When available, tree kangaroos will eat eggs, birds, and small animals. Although tree kangaroos aren't strict vegetarians, terrestrial kangaroos are only herbivores. While these two animals are related, they aren't quite the same.
It can be assumed that the question is talking about the common red kangaroo or even the wallaby which is a smaller 'version.' Either way, these terrestrial animals are strict vegetarians and because of this can consume very little water.
Most terrestrial Kangaroos are herbivorous, which makes them full time vegetarians they live mostly on grass, ferns, seeds and leaves but a group of tree Kangaroos also eat things they find on the tree which include sap, bark, insects, moss and baby birds. Kangaroos are ruminants animals like camels and cows, they regurgitate their food from their stomach before it is fully digested.
They have the ability to digest cellulose which is not common among mammals. It is also amazing to know that kangaroos, especially the terrestrial ones can survive on little water for a long time. They survive on the little water on the grass they chew. They can go several months without consuming much water.